How capture the info into a HTML checkbox?

I have created an HTML form which asks the user what types of marketing they are using. Depending on their answers, I intend to build a PHP recommender system based on those variations. I have started with a base function but it keeps returning the same echo statement regardless if I select none of the checkboxes or all of them. Here is the HTML code.

 <p>Marketing Mix:</p>    
 <input type="checkbox" name="TV" value="TV">TV<br>
 <input type="checkbox" name="Radio" value="Radio">Radio<br>
 <input type="checkbox" name="Search" value="Search">Search<br>
 <input type="checkbox" name="Mobile" value="Mobile">Mobile<br>
 <input type="checkbox" name="Mail" value="Mail">Direct Mail<br>
 <br>

And here is the PHP function to process the HTML form.

 function adMix () {
 $tv = (isset($_POST['TV']));
 $radio = (isset($_POST['Radio']));
 $search = (isset($_POST['Search']));
 $mobile = (isset($_POST['Mobile']));
 $mail = (isset($_POST['Mail']));
 if ($tv || $radio || $search || $mobile || $mail) {
     echo "You are running a comprehensive ad campaign.";
 } else {
     echo "You can do more to market your dealership.";
    }
 }

Any help would be greatly appreciated. I've tried coding this six ways to Sunday but it's been a while since I wrote in PHP and just can't figure it out.